Dramatic improvements in the classification of non-Hodgkin's lymphoma create an exceptional opportunity to investigate the causes, prevention, and course of a common subtype, marginal zone lymphoma (MZL). We propose (1) a case-control study and (2) a case series set in the KP population. We will identify 250 cases of MZL among KP members diagnosed during the period 1996 through June, 2003. A random sample of the general KP population will be identified as a control group to achieve a case-control ratio of 1:1. Information from the medical record and computerized databases will be obtained. The specific aims are as follows: 1. We will explore hypotheses that a history of chronic infectious and inflammatory diseases increases the risk of MZL. 2. We will obtain pilot data to assess (a) the quality of the pathologic diagnosis and (b) the prevalence of past medical history in cases and controls for a future, large-scale epidemiologic study. 3. Among the cases, we will describe the course and management of MZL to characterize prognosis and identify areas for future clinical research. The key strengths of the study are the population-based setting and the focus on MZL. In addition, the chart is a highly accurate and complete source of prospective information on medical exposures, disease course, and disease management. It is anticipated that 75% of the cases will have 10 years or longer enrollment in the health plan preceding their diagnosis, with correspondingly complete information in the medical record. The proposed study will explore important hypotheses, provide pilot data needed to develop a large-scale study, identify prognostic factors to give patients and providers the crucial information they need to make lifestyle and treatment decisions, and examine variation in care to focus future clinical research.